1. Field of the Invention
The present invention relates to an amplifier and more specifically to a control device of an amplifier for amplifying a signal having a property of a burst.
2. Description of the Related Art
FIG. 1 shows an image of a signal (burst signal) having a property of a burst. As shown in FIG. 1, a burst signal is placed in a transmission ON status after a transmission OFF period, and placed in the transmission OFF status again after the transmission ON period. The transmission ON period starts in a burst manner.
When a TDD (time division duplex) signal, etc. having a property of a burst is power-amplified by an amplifier, a distortion (over shoot) occurs on an amplified signal by the transient response of an amplifier power supply system immediately after the signal is input (transmission ON period) as shown in FIG. 2. When a burst signal is switched from the transmission ON status to the transmission OFF status, a distortion (under shoot) occurs on an amplified signal by the transient response of an amplifier power supply system as shown in FIG. 2.
The distortion of the amplification output signal of the amplifier incurs the degradation of a transmission signal, and appears as a power leak to the outside of the frequency band of the transmission signal, that is, the degradation of an ACLR (adjacent channel leakage ratio) characteristic, thereby indicating an interference signal to a adjacent channel.
Normally, when an amplifier is used with high power efficiency, it is necessary to operate an amplifier by a high bias setting. At this time, a low idle current flows through the amplifier during a no signal period. In this status, when a transmission signal is suddenly input (when the signal is suddenly placed in the transmission ON status), the current of the amplifier flows at a high speed. The sudden current change allows the power supply system to indicate a transient response, consequently changes the amplifier characteristic, and incurs the distortion of the amplification signal of the amplifier.
FIG. 3 shows a change in the current flowing through the amplifier when a transmission signal having a property of a burst is placed in the transmission ON or OFF status.
When the transmission signal is in the transmission OFF period, an idle current 111 flows through the amplifier. In the transmission ON period, the current flowing through the amplifier fluctuates in the range of a hatch rectangle 113 shown in FIG. 3, and an average of a current (average current of a transmission signal) 115 indicated by the broken lines shown in FIG. 3.
One of the methods for reducing the distortion generated when a TDD signal, etc. having a property of a burst is amplified can be a technique of performing a ramping process on an amplifier input signal (refer to, for example, Japanese Published Patent Application No. H5-129860, Japanese Published Patent Application No. H10-23089, etc.). In these techniques, a variable ATT (variable attenuator) is performed at a stage preceding the amplifier, and a burst signal is gradually boosted (or attenuated).
When the ramping process is performed on the amplifier input signal using the variable ATT, and the distortion of the amplifier amplification signal due to the property of a burst, the amplitude of the amplifier input signal is directly processed. This refers to changing and outputting a transmission signal, thereby degrading signal quality.